A lithographic printing plate is a printing plate which is used for lithographic printing. The surface of the printing plate consists of image areas to which an ink is received and non-image areas which do not receive an ink and repel. In general, the non-image areas form a surface where water is adsorbed and kept thereon. In an actual printing process, the non-image areas is firstly provided with water to repel an ink. Thus, in a lithographic printing plate, the non-image areas are made of a hydrophilic material whose characteristics influence greatly the performance of the lithographic printing plate.
A printing plate most widely used as a lithographic printing plate is generally called as “PS-plate”. The PS-plate uses an anode-oxidized aluminum plate whose surface is sand-blasted as a hydrophilic material, and a layer of an ink-receiving photosensitive material is formed on the anode-oxidized aluminum plate. The PS-plate is exposed to light corresponding to an original image, and the photosensitive material except for those in image areas is eliminated to expose the underlying hydrophilic aluminum surface so that an image consisting of image areas and non-image areas is formed on the printing plate.
The process in which the photosensitive layer is eliminated after exposure to light is called as development. Because in the development process, generally an alkaline solution or an organic solvent is used as a developer, disposal of the developer waste has loaded a heavy burden on printing operation. Particularly in recent years, with increasing awareness of environmental problems, it is requested for the developer waste to be disposed in environmentally completely harmless state. In addition, an exclusive apparatus is required for development. From the standpoint of the problems of high installation cost of such apparatus or requirement of keeping sufficient working space, a printing plate that does not require development process is requested.
As a plate that does not require development process, for example, a plate whose non-image areas comprises a hydrophilic swelling layer is disclosed in Patent Document 1 (Japanese Patent Laid-open publication No. 282142/1996). This plate, after the hydrophilic swelling layer is formed, acquires photosensitivity by absorbing photosensitive materials in the hydrophilic swelling layer. The image areas lose hydrophilicity as the result of reaction of the photosensitive materials contained in the hydrophilic swelling layer through exposure to light. However, in this method, unnecessary photosensitive materials remain in the non-image areas, so that the problem of requiring a rinsing process still remains, where the photosensitive materials in the non-image areas are rinsed off after exposure to light.
Patent Document 2 (Japanese Patent Laid-open publication No. 314934/1995) discloses a plate having a constitution where an ink-repelling layer made of a silicone resin is superimposed on an inorganic light-absorbing layer of titanium or titanium oxide and the like. In this plate, the silicone resin layer repels an ink to form non-image areas, and image areas are formed by irradiation of near-infrared light. On printing, the silicone resin layer is eliminated after irradiation with light so as to expose the support surface having affinity to ink. In order to eliminate the silicone resin completely, wiping off operation is required. When the silicone resin is wiped off incompletely, the ink is not sufficiently adhered to the irradiated portion and defects are formed in the image areas, leading to a drawback of imperfect printing.
Further, Patent Document 3 (Japanese Patent Laid-open publication No. 109141/2001) discloses a printing plate having a hydrophilic layer to which a water-soluble polymer compound is added. This plate employs a so-called ablation mechanism in which the hydrophilic layer is irradiated with laser beam to cause, by thermal energy, chemical reactions such as burning, dissolution, decomposition, vaporization or explosion or physical changes in the irradiated portion of the hydrophilic layer, and as a result, the adhesion of the irradiated portion of the hydrophilic layer is lowered so as to eliminate selectively the portion irradiated with laser beam. This method still has such a problem of requiring a wiping off operation with a water-swollen pad and the like.
In recent years, a method called on-press development has been proposed, in which development process is carried out in printing machines on site. In the on-press development method, unnecessary portion after exposure to light is eliminated in printing machines, without using developer, with a fountain solution or by mechanical contact with an impression drum or a blanket drum in the machine. For example, Patent Document 4 (Japanese Patent Laid-open publication No. 127683/1997) and Patent Document 5 (Japanese Patent Laid-open publication No. 171249/1997) disclose plates having a hydrophilic support with a photosensitive layer containing water-dispersing thermoplastic resin particles that become to have an affinity to ink by melting and welding through exposure to light. The photosensitive layer of the plates can be developed with a fountain solution in printing machines on site, because the unexposed portion is dissolved in water. The on-press development method has an advantage of having no requirement for installation of a development apparatus and actual operation of development, however, the method has such drawbacks of requiring strict humidity control for the plate as well as contaminating the fountain solution and ink with insoluble components after exposure to light.
As a plate which does not require both wet-development and on-press development, Patent Document 6 (Japanese Patent Laid-open publication No. 1850/1995) discloses a method in which a photosensitive layer contains microcapsules encapsulating an oleophilic substance in a hydrophilic resin which reacts with the hydrophilic groups of the hydrophilic resin and forms chemical bonds, and the microcapsules are collapsed by irradiation with light so as to change the hydrophilic resin into oleophilic. However, this method also has a disadvantage of requiring microcapsules with small diameter to meet the request for higher resolution and lower scumming and also has a difficulty in the production of such microcapsules. While the microcapsules can be collapsed relatively uniformly by heat and pressure on printing with thermal heads, there is such a problem that the microcapsules are collapsed non-uniformly by irradiation with light on printing and also have poor resolution.
The present applicants disclose a plate for lithographic printing in Patent Document 7 (International Publication WO 01/083234), in which the plate is consisted of a photosensitive layer of a hydrophilic resin which is obtained by cross-linking a photosensitive resin composition containing a hydrophilic polymer, a cross-linking agent, and a photo-absorber, or another photosensitive resin composition containing a hydrophilic polymer, across-linking agent, a photo-absorber, and a hydrophobic polymer, and the surface of the plate changes from hydrophilic to inkphilic (having affinity to ink) by irradiation with light. This plate requires no development and wiping off operation and has a good sensitivity and resolution because changes occur only on the surface of the light-irradiated portion. However, an improvement is awaited so as to completely eliminate the ink that adheres on non-image areas.
The same applicants disclose a lithographic printing plate in Patent Document 8 (Japanese Patent Laid-open publication No. 18547/2001), in which the plate has a hydrophilic layer comprising organic substances and has a porous surface. In the plate, an attempt of increasing hydrophilicity of the hydrophilic layer is made by increasing the surface area of the hydrophilic layer through its porous surface and increasing the water-absorbing rate to the plate. However, also in this plate, an improvement is desired so as to completely eliminate the ink that attached to non-image areas. Another improvement is also desired, because the plate does not provide satisfactory performance of imaging when an image is directly printed on the plate by irradiation with laser beam.                Patent Document 1: Japanese Patent Laid-open publication No. 282142/1996        Patent Document 2: Japanese Patent Laid-open publication No. 314934/1995        Patent Document 3: Japanese Patent Laid-open publication No. 109141/2001        Patent Document 4: Japanese Patent Laid-open publication No. 127683/1997        Patent Document 5: Japanese Patent Laid-open publication No. 171249/1997        Patent Document 6: Japanese Patent Laid-open publication No. 1850/1995        Patent Document 7: International Publication WO 01/083234        Patent Document 8: Japanese Patent Laid-open publication No. 18547/2001        